Abstract Aims.Numerous spectroscopic observations provide compelling evidence
for a non-canonical mixing process that modifies the surface abundances of
Li, C and N of low-mass red giants when they reach the bump in the luminosity function.
Eggleton and collaborators have proposed that a molecular weight inversion created by the
3He(3He, 2p)4He reaction may be at the origin of this mixing,
and relate it to the Rayleigh-Taylor instability.
We argue that one is actually dealing with a double
diffusive instability referred to as thermohaline convection and
we discuss its influence on the red giant branch.Methods.We compute stellar models of various initial metallicities that include thermohaline
mixing, which is treated as a diffusive process based on the prescription
given originally by Ulrich for the turbulent diffusivity produced by the
thermohaline instability in stellar radiation zones.Results.Thermohaline mixing simultaneously accounts for the observed behaviour
of the carbon isotopic ratio and of the abundances of Li, C and N in
the upper part of the red giant branch.
It significantly reduces the 3He production with respect to canonical
evolution models as required by measurements of 3He/H in galactic HII regions.Conclusions.Thermohaline mixing is a fundamental physical process that must be included
in stellar evolution modeling.